40 CFR Part 50, Appendix A-1 to Part 50 - Reference Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method)

Appendix A-1 to Part 50—Reference Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method)

1.0Applicability

1.1This ultraviolet fluorescence (UVF) method provides a measurement of the concentration of sulfur dioxide (SO2) in ambient air for determining compliance with the national primary and secondary ambient air quality standards for sulfur oxides (sulfur dioxide) as specified in § 50.4, § 50.5, and § 50.17 of this chapter. The method is applicable to the measurement of ambient SO2 concentrations using continuous (real-time) sampling. Additional quality assurance procedures and guidance are provided in part 58, appendix A, of this chapter and in Reference 3.

2.0Principle

2.1This reference method is based on automated measurement of the intensity of the characteristic fluorescence released by SO2 in an ambient air sample contained in a measurement cell of an analyzer when the air sample is irradiated by ultraviolet (UV) light passed through the cell. The fluorescent light released by the SO2 is also in the ultraviolet region, but at longer wavelengths than the excitation light. Typically, optimum instrumental measurement of SO2 concentrations is obtained with an excitation wavelength in a band between approximately 190 to 230 nm, and measurement of the SO2 fluorescence in a broad band around 320 nm, but these wavelengths are not necessarily constraints of this reference method. Generally, the measurement system (analyzer) also requires means to reduce the effects of aromatic hydrocarbon species, and possibly other compounds, in the air sample to control measurement interferences from these compounds, which may be present in the ambient air. References 1 and 2 describe UVF method.

2.2The measurement system is calibrated by referencing the instrumental fluorescence measurements to SO2 standard concentrations traceable to a National Institute of Standards and Technology (NIST) primary standard for SO2 (see Calibration Procedure below).

2.3An analyzer implementing this measurement principle is shown schematically in Figure 1. Designs should include a measurement cell, a UV light source of appropriate wavelength, a UV detector system with appropriate wave length sensitivity, a pump and flow control system for sampling the ambient air and moving it into the measurement cell, sample air conditioning components as necessary to minimize measurement interferences, suitable control and measurement processing capability, and other apparatus as may be necessary. The analyzer must be designed to provide accurate, repeatable, and continuous measurements of SO2 concentrations in ambient air, with measurement performance as specified in Subpart B of Part 53 of this chapter.

2.4Sampling considerations: The use of a particle filter on the sample inlet line of a UVF SO2 analyzer is required to prevent interference, malfunction, or damage due to particles in the sampled air.

3.0Interferences

3.1The effects of the principal potential interferences may need to be mitigated to meet the interference equivalent requirements of part 53 of this chapter. Aromatic hydrocarbons such as xylene and naphthalene can fluoresce and act as strong positive interferences. These gases can be removed by using a permeation type scrubber (hydrocarbon “kicker”). Nitrogen oxide (NO) in high concentrations can also fluoresce and cause positive interference. Optical filtering can be employed to improve the rejection of interference from high NO. Ozone can absorb UV light given off by the SO2 molecule and cause a measurement offset. This effect can be reduced by minimizing the measurement path length between the area where SO2 fluorescence occurs and the photomultiplier tube detector (e.g., <5 cm). A hydrocarbon scrubber, optical filter and appropriate distancing of the measurement path length may be required method components to reduce interference.

4.1Apparatus: Figure 2 shows a typical generic system suitable for diluting a SO2 gas cylinder concentration standard with clean air through a mixing chamber to produce the desired calibration concentration standards. A valve may be used to conveniently divert the SO2 from the sampling manifold to provide clean zero air at the output manifold for zero adjustment. The system may be made up using common laboratory components, or it may be a commercially manufactured system. In either case, the principle components are as follows:

4.1.1SO2 standard gas flow control and measurement devices (or a combined device) capable of regulating and maintaining the standard gas flow rate constant to within ±2 percent and measuring the gas flow rate accurate to within ±2, properly calibrated to a NIST-traceable standard.

4.1.2Dilution air flow control and measurement devices (or a combined device) capable of regulating and maintaining the air flow rate constant to within ±2 percent and measuring the air flow rate accurate to within ±2, properly calibrated to a NIST-traceable standard.

4.1.3Mixing chamber, of an inert material such as glass and of proper design to provide thorough mixing of pollutant gas and diluent air streams.

4.1.4Sampling manifold, constructed of glass, polytetrafluoroethylene (PTFE TeflonTM), or other suitably inert material and of sufficient diameter to insure a minimum pressure drop at the analyzer connection, with a vent designed to insure a minimum over-pressure (relative to ambient air pressure) at the analyzer connection and to prevent ambient air from entering the manifold.

4.1.5Standard gas pressure regulator, of clean stainless steel with a stainless steel diaphragm, suitable for use with a high pressure SO2 gas cylinder.

4.1.6Reagents

4.1.6.1SO2 gas concentration transfer standard having a certified SO2 concentration of not less than 10 ppm, in N2, traceable to a NIST Standard Reference Material (SRM).

4.1.6.2Clean zero air, free of contaminants that could cause a detectable response or a change in sensitivity of the analyzer. Since ultraviolet fluorescence analyzers may be sensitive to aromatic hydrocarbons and O2-to-N2 ratios, it is important that the clean zero air contains less than 0.1 ppm aromatic hydrocarbons and O2 and N2 percentages approximately the same as in ambient air. A procedure for generating zero air is given in reference 1.

4.2Procedure

4.2.1Obtain a suitable calibration apparatus, such as the one shown schematically in Figure 1, and verify that all materials in contact with the pollutant are of glass, TeflonTM, or other suitably inert material and completely clean.

4.2.3Ensure that there are no leaks in the system and that the flow measuring devices are properly and accurately calibrated under the conditions of use against a reliable volume or flow rate standard such as a soap-bubble meter or a wet-test meter traceable to a NIST standard. All volumetric flow rates should be corrected to the same reference temperature and pressure by using the formula below:

Where:

Fc = corrected flow rate (L/min at 25 °C and 760 mm Hg),

Fm = measured flow rate, (at temperature, Tm and pressure, Pm),

Pm = measured pressure in mm Hg, (absolute), and

Tm = measured temperature in degrees Celsius.

4.2.4Allow the SO2 analyzer under calibration to sample zero air until a stable response is obtained, then make the proper zero adjustment.

4.2.5Adjust the airflow to provide an SO2 concentration of approximately 80 percent of the upper measurement range limit of the SO2 instrument and verify that the total air flow of the calibration system exceeds the demand of all analyzers sampling from the output manifold (with the excess vented).

4.2.6Calculate the actual SO2 calibration concentration standard as:

Where:

C = the concentration of the SO2 gas standard

Fp = the flow rate of SO2 gas standard

Ft = the total air flow rate of pollutant and diluent gases

4.2.7When the analyzer response has stabilized, adjust the SO2 span control to obtain the desired response equivalent to the calculated standard concentration. If substantial adjustment of the span control is needed, it may be necessary to re-check the zero and span adjustments by repeating steps 4.2.4 through 4.2.7 until no further adjustments are needed.

4.2.8Adjust the flow rate(s) to provide several other SO2 calibration concentrations over the analyzer's measurement range. At least five different concentrations evenly spaced throughout the analyzer's range are suggested.

4.2.9Plot the analyzer response (vertical or Y-axis) versus SO2 concentration (horizontal or X-axis). Compute the linear regression slope and intercept and plot the regression line to verify that no point deviates from this line by more than 2 percent of the maximum concentration tested.

Note:

Additional information on calibration and pollutant standards is provided in Section 12 of Reference 3.

5.0Frequency of Calibration

The frequency of calibration, as well as the number of points necessary to establish the calibration curve and the frequency of other performance checking will vary by analyzer; however, the minimum frequency, acceptance criteria, and subsequent actions are specified in Reference 3, Appendix D: Measurement Quality Objectives and Validation Template for SO2 (page 9 of 30). The user's quality control program should provide guidelines for initial establishment of these variables and for subsequent alteration as operational experience is accumulated. Manufacturers of analyzers should include in their instruction/operation manuals information and guidance as to these variables and on other matters of operation, calibration, routine maintenance, and quality control.

Comments must be received on or before April 6, 2015. Public Hearings: If, by January 26, 2015, the EPA receives a request from a member of the public to speak at a public hearing concerning the proposed decision, we will hold a public hearing, with information about the hearing provided in a subsequent notice in the Federal Register .

40 CFR Part 50

Summary

Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and the national ambient air quality standards (NAAQS) for lead (Pb), the EPA is proposing to retain the current standards, without revision.

The public hearings will be held on January 29, 2015, in Washington, DC and Arlington, Texas, and on February 2, 2015, in Sacramento, California. Please refer to SUPPLEMENTARY INFORMATION for additional information on the public hearings.

40 CFR Parts 50, 51, 52, 53, and 58

Summary

The Environmental Protection Agency (EPA) is announcing three public hearings for the proposed rule titled, “National Ambient Air Quality Standards for Ozone,” that was published in the Federal Register on December 17, 2014. The hearings will be held in Washington, DC, Arlington, Texas, and Sacramento, California. Based on its review of the air quality criteria for ozone (O 3 ) and related photochemical oxidants and national ambient air quality standards (NAAQS) for O 3, the EPA proposes to make revisions to the primary and secondary NAAQS for O 3 to provide requisite protection of public health and welfare, respectively. The EPA is proposing to revise the primary standard to a level within the range of 0.065 to 0.070 parts per million (ppm), and to revise the secondary standard to within the range of 0.065 to 0.070 ppm, which air quality analyses indicate would provide air quality, in terms of 3-year average W126 index values, at or below a range of 13-17 ppm-hours. The EPA proposes to make corresponding revisions in data handling conventions for O 3 and conforming changes to the Air Quality Index; to revise regulations for the Prevention of Significant Deterioration program to add a transition provision for certain applications; and to propose schedules and convey information related to implementing any revised standards. The EPA is proposing changes to the O 3 monitoring seasons, the Federal Reference Method (FRM) for monitoring O 3 in the ambient air, Federal Equivalent Method procedures for testing, and the Photochemical Assessment Monitoring Stations network. Along with proposing exceptional event schedules related to implementing any revised O 3 standards, the EPA is proposing to apply this same schedule approach to other future revised NAAQS and to remove obsolete regulatory language for expired exceptional event deadlines. The EPA is proposing to make minor changes to the procedures and time periods for evaluating potential FRMs and equivalent methods (including making the requirements for nitrogen dioxide consistent with the requirements for O 3 ) and to remove an obsolete requirement for the annual submission of documentation by manufacturers of certain particulate matter monitors.

Written comments on this proposed rule must be received by March 17, 2015. Public Hearings: The EPA intends to hold three public hearings on this proposed rule in January 2015. These will be announced in a separate Federal Register notice that provides details, including specific dates, times, addresses, and contact information for these hearings.

40 CFR Parts 50, 51, 52, 53, and 58

Summary

Based on its review of the air quality criteria for ozone (O 3 ) and related photochemical oxidants and national ambient air quality standards (NAAQS) for O 3, the Environmental Protection Agency (EPA) proposes to make revisions to the primary and secondary NAAQS for O 3 to provide requisite protection of public health and welfare, respectively. The EPA is proposing to revise the primary standard to a level within the range of 0.065 to 0.070 parts per million (ppm), and to revise the secondary standard to within the range of 0.065 to 0.070 ppm, which air quality analyses indicate would provide air quality, in terms of 3-year average W126 index values, at or below a range of 13-17 ppm-hours. The EPA proposes to make corresponding revisions in data handling conventions for O 3 and conforming changes to the Air Quality Index (AQI); to revise regulations for the prevention of significant deterioration (PSD) program to add a transition provision for certain applications; and to propose schedules and convey information related to implementing any revised standards. The EPA is proposing changes to the O 3 monitoring seasons, the Federal Reference Method (FRM) for monitoring O 3 in the ambient air, Federal Equivalent Method (FEM) procedures for testing, and the Photochemical Assessment Monitoring Stations (PAMS) network. Along with proposing exceptional event schedules related to implementing any revised O 3 standards, the EPA is proposing to apply this same schedule approach to other future revised NAAQS and to remove obsolete regulatory language for expired exceptional event deadlines. The EPA is proposing to make minor changes to the procedures and time periods for evaluating potential FRMs and equivalent methods (including making the requirements for nitrogen dioxide consistent with the requirements for O 3 ) and to remove an obsolete requirement for the annual submission of documentation by manufacturers of certain particulate matter monitors. For additional information, see the Executive Summary, section I.A.